Release v2.0.0

• BREAKING: drop support for Node.js 8, 9, 10, and 11. Support for Node.js 12
  LTS and Node.js 14+.
• BREAKING: use dynamic imports (lazy-loading) to improve parallel processing
  (requires Node.js 10+).
• BREAKING: use worker_threads for yUML and PlantUML rendering (requires Node.js
  12+).
• BREAKING: use unflagged ESM (requires Node.js 12.17+).
• Can now read Markdown from stdin.
• No more child process spawning.
• Remove use of paths relative to cwd.
• Export md2html function for external API usage.
• Improve SVG error visibility.
• Update dependencies.